In this collection of essays, written over the past decade, Robert Cummings Neville addresses contemporary debates about the concept of religion and the importance of the comparative method in theology, while advancing and defending his own original definition of religion. Neville's hypothesis is that religion is a cognitive, existential, and practical engagement of ultimate realities—five ultimate conditions of existence that need to be engaged by human beings. The essays, which range from formal articles to invited lectures, develop this hypothesis and explore its ramifications in religious experience, philosophical theology, religious studies, and the works of important thinkers in philosophy of religion. Defining Religion is an excellent introduction to Neville's work, especially to the systematic philosophical theology presented in his magisterial three-volume set Philosophical Theology.